Lithium ion batteries (LIBs) have attracted a great deal of attention as potential replacements for conventional gasoline- or diesel-powered internal combustion engines. As an improved anode material for LIBs, lithium titanate oxide, Li4Ti5O12 (LTO) has good characteristics in terms of safety because of its stable structure during charging/discharging and its inertness towards the formation of solid-electrolyte interphase (SEI) on LTO surface.
Furthermore, LTO is a promising anode material for certain niche applications requiring high rate capability and long cycle life, and offers advantages in terms of power and chemical stability. A disadvantage of LTO is its poor electronic conductivity that limits its full capacity at high charge-discharge rates. The increase in electronic conductivity of LTO can facilitate higher-rate operability of LTO anodes.
It is well-known in the art that shorter diffusion lengths for Li+ ions and electronic transport in nanoparticles improve the high-rate performance of lithium battery anodes. The morphology of LTO has also been regarded as a critical factor in lithium intercalating activity and cycling stability of the electrode materials.
Most of the conventional LTO materials are now synthesized by solid reaction method which needs to be heated at high temperature (typically 700-900° C.) for a long time, and the method is energy-consuming. What is more, this method fails to control the particle size and morphology. On the other hand, nanomaterials can be fabricated using various template precursors, such as AAO, triblock copolymers, porous silica and polystyrene spheres. These preparation methods often require removal of the templates after synthesis that may damage the desired configuration of the nanomaterials and make the synthesis process become more complicated.
As an improved anode material for LIBs, LTO shows extremely flat discharge and charge plateaus at about 1.55 V (vs. Li/Li+) and zero-strain insertion characteristics, as well as excellent lithium ion mobility, so LTO entirely eliminate potential safety issues and exhibit excellent cycling performance. Nevertheless, its low electronic conductivity (<10−11 S/m) results in poor rate performance. Many approaches have been developed to overcome its poor electronic conductivity, such as surface coatings with conductive material.
Recently, nanostructured LTO is expected to exhibit improved rate performance because of the shorter transport path lengths of lithium ions and electrons. The mesoporous structured materials offer fast ion/electron transfer and sufficient contact interface between active materials and electrolyte, resulting in high Li storage capacity and high rates of insertion.
Novel mesoporous structured materials constituted by “nano-size” LTO combined with “coating with conductive carbon” are ideal materials possessing rapid electronic and ionic transport.
However, the conductivity of the carbon coated LTO material is still not high enough, and the fabrication of nanosized particles with a mesoporous structure is still a challenge through using solid state reaction or sol-gel methods. Recently, the solution-phase route has been regarded as a feasible way to control the particle size and shape.
Conventionally, lithium titanate is obtained from titanium dioxide as a starting material. In order to prepare an electrode material having excellent characteristics, titanium dioxide, as a starting material of lithium titanate, plays an important role. Between a lithium precursor and a titanium precursor, which are the starting materials of lithium titanate, more considerations are needed for the titanium precursor in terms of the type and amount of an element used, and the corresponding composition ratio since the titanium precursor affects battery performance more than those of the lithium precursor. When high-purity titanium dioxide is used as a precursor of lithium titanate, many problems including high manufacturing costs may occur.
CN102610824 discloses a method for preparing nano-scaled LTO/Ag composite for anode material of the lithium-ion battery. The Ag doping is simultaneously carried out to modify the nano-scaled LTO to improve the conductivity of the lithium titanate. The chemical ingredients and the grain diameters of the lithium titanate are controlled through hydrothermal treatment, for reducing the temperature during preparation, and preventing the grain agglomeration. Nevertheless, the Ag is usually not well distributed within the lithium titanate, leading to low electrical conductivity.
CN103022461 discloses a rare-earth metal doped micro-nanoscaled lithium titanate anode material for providing large rate discharge characteristic. The molecular formula of the lithium titanate is LixMpTiyOz, in which M represents doped rare-earth metal ions. By adopting spherical titanium dioxide as an initial raw material and using water or ethanol as reaction agent, the preparation method is achieved by hydrothermal reaction and calcination. Nevertheless, the rare-earth materials are expensive, thereby increasing substantially the manufacturing cost.
Consequently, there is an unmet need for a LTO material providing high conductivity for increasing the capacity at high charging/discharging rates, and high energy storage capacity.